The principal toxic sulfur-containing gases in waste gas streams are hydrogen sulfide (H.sub.2 S) and sulfur dioxide (SO.sub.2). Present to a lesser extent--but also toxic--are carbonyl sulfide (COS) and carbon disulfide (CS.sub.2).
The Claus process has long been utilized to remove hydrogen sulfide from gas streams by reacting hydrogen sulfide with sulfur dioxide in the presence of a catalyst, such as bauxite, alumina, cobalt molybdates, etc., to form sulfur and water. Alternatively, if the CO.sub.2 content of the gas stream is low and the H.sub.2 S content is high, one partially burns the H.sub.2 S in air to form sulfur, water and SO.sub.2. This is then followed by the H.sub.2 S+SO.sub.2 reaction in a subsequent process stage. The H.sub.2 S may be present in concentrations ranging from a few parts per million up to about 80 percent, depending upon the type of process or process stage from which the gases emanate. Processes which emit this pollutant include coal gasifiers, coal liquification plants, smelters, power plants, ore roasting units, and chemical manufacturing plants.
The Claus process has had three serious deficiencies. First, the process requires a "balancing" of the H.sub.2 S and SO.sub.2 to form the stoichiometric amounts of each required to balance the reaction, i.e., 2 parts of H.sub.2 S to 1 part of SO.sub.2. This balancing is accomplished by oxidizing part of the H.sub.2 S to form SO.sub.2. Secondly, several stages of catalyst are usually required to obtain the present substantial degree of reduction necessary via: EQU 2H.sub.2 S+SO.sub.2 .fwdarw.2H.sub.2 O+3/2S.sub.2
in order to meet pollution standards. Thirdly, the presence of excessive amounts of oxygen deactivates most of the catalysts suggested for use in this process, thereby destroying the commercial feasibility thereof.
Therefore, from a conceptual point of view, an inherently better process would be one in which the oxygen is used to oxidize the H.sub.2 S directly to elemental sulfur without having to generate additional SO.sub.2. Although this catalytic oxidation of H.sub.2 S to elemental sulfur is known [See, for example, Sreeramamurthy et al., Journal of Catalysis 37, 287-296(1975)], there are no catalysts, to the best of our knowledge, which are commercially utilized to oxidize H.sub.2 S by the direct use of oxygen (principally because the catalysts are deactivated in the presence of oxygen). It would, therefore, be desirable to have a process for the catalytic oxidation by oxygen of H.sub.2 S to elemental sulfur using a catalyst which is not deactivated by oxygen.
In view of increasingly stringent requirements on removal of toxic sulfur compounds in gas streams, there also exists a need for the development of catalytic processes which would reduce the concentrations of COS and CS.sub.2 as well as H.sub.2 S and SO.sub.2, to acceptable limits. During the course of the H.sub.2 S+SO.sub.2 reaction shown above, low molecular weight hydrocarbons, when present in the gas stream, react with the sulfur components and lead to the formation of COS and CS.sub.2. In addition, the gas station may already contain small amounts of COS and CS.sub.2 and these noxious compounds are not removed during the course of the Claus reaction using ordinary catalysts operating at low temperatures.
It is, therefore, a primary object of this invention to provide a novel process for the removal of hydrogen sulfide from gas streams containing hydrogen sulfide.
Another primary object of this invention is to provide a novel process for the simultaneous removal of both sulfur dioxide and hydrogen sulfide from gaseous waste streams.
A further primary object of this invention is to provide a novel process for the removal of carbonyl sulfide and carbon disulfide in conjunction with the removal of hydrogen sulfide from gas streams containing hydrogen sulfide.
It is another object of this invention to provide a process for the concomitant oxidation of hydrogen sulfide and reduction of sulfur dioxide using specific catalytic compositions which are not subject to poisoning by oxygen or water, and are less subject to the aforementioned deficiencies.
It is further additional object of this invention to provide a process for the catalytic reaction of hydrogen sulfide or carbonyl sulfide and sulfur dioxide to form elemental sulfur using specific catalytic materials which are not subject to poisoning by oxygen.
Yet a still further object of this invention is to provide novel catalytic compositions suitable for removal of toxic sulfur gases from gas streams.
And another object of this invention is to provide novel catalytic compositions suitable for use in the processes of this invention.
These and still further objects, advantages and features of the present invention will become apparent upon consideration of the following disclosure.